It is known in the prior art to control the movement of one or more vehicles coupled together to form a train through a fixed block track circuit signaling system. Specific signal blocks of track are established by predetermined low impedance electrical signal boundaries at the ends of each signal block. When a train vehicle is present in a given signal block, at least one vehicle axle of the train electrically shorts between the two conductive track rails on which the vehicle runs. A signal transmitter is coupled to the track at one end of each signal block and a cooperative signal receiver is coupled to the track at the opposite end of that same signal block, for providing desired control of the train movement and detecting the occupancy of a train vehicle within that signal block. The train position is detected electrically as the individual vehicles of the train move along the track rails, passing through succeeding blocks, as described in U.S. Pat. No. R.E. 27,472 of G. M. Thornebooth and as described in an article published in the Westinghouse Engineer for September 1972 at pages 145 to 151.
The vehicle detection equipment is located at the wayside of the track, and when a vehicle is detected in a given signal block, a control signal is provided to influence the speed code of the next previous block and if desired this control signal can ripple back to one or more previous signal blocks. Under certain abnormal operational conditions such as when electric power to the train vehicle is lost or when a corrosion film or ice builds up on the top of the track rails, there is a small possibility that the conventional signaling system may not detect a train vehicle within a present signal block. The conventional signaling system detects the train vehicle shunt impedance, and if this is abnormally high for some reason the train vehicle occupancy detection becomes difficult. An excessive corrosion film will increase the train vehicle shunt impedance by forming a barrier layer to make more difficult the detection of train vehicle occupancy.
The need for sensing train vehicle presence in a given signal block has led to occupancy detection and sequential occupancy release control of train movement, such that when a vehicle occupancy is detected in a given signal block it is necessary to subsequently detect occupancy in the next succeeding signal block before a release is desired of the occupancy behind the vehicle. The occupancy in a previous signal block is retained and not released until the train vehicle is positively detected in the next signal block.
In conventional wayside train control systems the detection of train movement is primarily a passive operation. Every effort is made to insure the reliable and consistent detection of train positions under the widest possible range of conditions. Typical railroad interlocking procedures rarely employ any means of retaining or latching the last known position of a train. One prior art solution to this problem has been the check-in and check-out principle whereby the presence of a train once detected in a given track circuit block is remembered until the same train is detected in the next adjacent track circuit block. This sequential release of last occupancies when accomplished by means of hard-wired logic systems has proved to be complex and cumbersome. In addition consideration should be given to traffic direction, switch position, gate status, block lengths and station and system boundary conditions, all of which are unique to each individual track circuit signal block, such that the amount of customized hardware circuitry necessary to accomplish the above train control arrangement can be rather expensive.
An article in Business Week for Mar. 2, 1974 at page 51, discusses a control system for providing occupancy detection and sequential occupancy release control of train vehicle movement for a transit system. In relation to successive signal blocks N-2, N-1, N, N+1 and N+2 and so forth, the following train movement control algorithms can be utilized: EQU Q.sub.N = SQ.sub.N = Q.sub.N.sub.-1 [Q.sub.N.sub.+1 .sup.. I.sub.N.sub.+1 (B.sub.N.sub.+1 + Q.sub.N.sub.+2 .sup.. I.sub.N.sub.+2 (B.sub.N.sub.+2 + etc. (1) EQU Q.sub.N = RQ.sub.N = Q.sub.N.sub.+1 ( 2) EQU b.sub.n = sb.sub.n = i.sub.n .sup.. q.sub.n .sup.. q.sub.n.sub.-1 [q.sub.n.sub.-2 (b.sub.n.sub.-1 + q.sub.n.sub.-3 (b.sub.n.sub.-2 + etc. (3) EQU B.sub.N = RB.sub.N = I.sub.N ( 4) EQU o.sub.n = so.sub.n = i.sub.n + q.sub.n ( 5)
where I.sub.N is the primary train vehicle occupancy indication signal for signal block N, Q.sub.N is the backup protection signal for block N, B.sub.N is the false or pseudo occupancy indication signal for signal block N and O.sub.N is the occupancy control signal for signal block N as seen by the primary train protection system. The O.sub.N signal controls the movement of a subsequent train vehicle when a train vehicle is detected within a given signal block N. The above equations (1) to (5) are operative with well known AND and OR logic relationships as indicated where the system is implemented in positive logic. The train control system is operative such that when the occupancy control signal O.sub.N is false this is applied to the previous signal block N-1 for permitting the normal speed code signal to be provided in signal block N-1 in relation to a train moving through block N-1, and indicates there is no train vehicle in signal block N to interfere with the movement of the train in the signal block N-1.
The occupancy control signal O.sub.N is set false when the occupancy indication signal I.sub.N for block N is false to indicate no train occupies signal block N and the backup protection signal Q.sub.N is false which indicates no previous train is present in signal block N and the next adjacent signal blocks in a forward direction of signal block N to interfere with the movement of a train within controlled signal block N-1. The above set Q.sub.N (Equation 1) is operative to set the backup protection signal Q.sub.N true when the signal Q.sub.N.sub.-1 is true and the protection signal Q.sub.N.sub.-1 is false and the occupancy indication signal I.sub.N.sub.+1 is true and the signal false or pseudo occupancy B.sub.N.sub.+1 is false, or the above set Q.sub.N (Equation 1) is operative to set the backup protection signal Q.sub.N true when the signal Q.sub.N.sub.-1 is true and the backup protection signal Q.sub.N.sub.+2 is false and the occupancy indication signal I.sub.N.sub.+2 is true and the false or pseudo occupancy signal B.sub.N.sub.+2 is true and so forth for all of the remaining track signal blocks within the Q line protection arrangement including signal block N up to a theoretical infinite number of signal blocks. The above reset Q.sub.N or (Q.sub.N) provided by above Equation (2) is operative to reset the backup protection signal Q.sub.N to false when the backup protection signal Q.sub.N.sub.+1 is true. The above set B.sub.N (Equation 3) is operative to set the false or pseudo occupancy signal B.sub.N to true when the occupancy indication signal I.sub.N is true and the backup protection signal Q.sub.N is false and the backup protection signal Q.sub.N.sub.+1 is false and the backup protection signal Q.sub.N.sub.+2 is false and the false or pseudo occupancy signal B.sub.N.sub.+1 is false or when the occupancy indication signal I.sub.N is true and the backup protection signal Q.sub.N is false and the backup protection signal Q.sub.N.sub.+1 is false and the backup protection signal Q.sub.N.sub.+3 is false and the false occupancy signal B.sub.N.sub.+2 is false, or and so forth as indicated by above Equation (3), for all of the remaining track signal blocks going backward from signal block N up to a theoretical infinite number of signal blocks. The reset B.sub.N (Equation 4) is operative to reset the false occupancy signal B.sub.N to false when the occupancy indication signal I.sub.N is false.
The above occupancy control signal O.sub.N (Equation 5) is operative with well known OR logic relationships to set the occupancy control signal O.sub.N to true when the occupancy indication signal I.sub.N is true or the backup protection signal Q.sub.N is true.